Large-screen, high-brightness electronic projection display apparatuses serve different broad areas of application: electronic presentations for e.g. business, education and advertising, entertainment for e.g. home theatre and electronic cinema, display of status and information for e.g. military, utilities and transportation, simulation in e.g. training and games. In all these areas the need to correctly display color images is large.
Electronic projection display apparatuses typically are based on one of three major technologies: CRT, liquid crystal display (LCD) technology or digital light processing (DLP) technology. Fixed format display methods include LCD and DLP technologies but exclude CRT. At the heart of a DLP projection display is provided a spatial light modulator (SLM) unit sometimes called a light valve unit. A spatial light modulator unit comprises at least one spatial light modulator, which is a device that modulates incident light in a spatial pattern corresponding to an electrical or optical input. The spatial light modulator is a pixelated device, i.e. an array of pixels display elements, wherein each pixel or group of pixels can be addressed and driven independently to project or display an arbitrary image. The spatial light modulator can modulate incident light in its phase, intensity, polarisation, or direction, and the light modulation may be achieved by a variety of materials exhibiting various electro-optic or magneto-optic effects or by materials that modulate light by surface deformation. An SLM comprises a one- or two-dimensional array of light-modulating elements. Silicon technology used in projection data monitors is capable of producing small-sized, two-dimensional light-valve arrays having several hundred thousand to several million light-modulating elements. Spatial light modulators are either transmissive or reflective. Transmissive devices modulate the light beam as it passes through the unit. Reflective devices modulate the light as it reflects from a mirror inside the unit. Typical examples of SLM devices are a deformable mirror device (DMD), also called digital mirror device or digital micromirror device, a reflective LCD device, an LCOS device, etc.
To obtain color projection systems basically three different types of projection systems are known : a single imager spatial color filter design, whereby each pixel is divided into three color subpixels; a single imager color field sequential system, whereby the beam is sequentially filtered to each primary color which is then modulated in the single imager, and a two, three or multi-imager parallel color system, whereby the light beam is split into two, three or more primary colors, each color beam is directed to an imager and the modulated color beams are converged into a single beam for projection. As the latter system allows the highest throughput efficiency, i.e. higher than three times the throughput efficiency of the other systems, the use of a three imager parallel color system is common.
Very often the three imager parallel color system uses an X-cube for color separation and recombination in the digital projection system. An X-cube is a dichroic prism which allows to split the white light of the light source into three primary colours and/or to recombine the three primary colours to one illumination beam for projection.
In order to obtain color images, either light of different sources having a different wavelength or wavelength range can be used, or, what is more common, the light of a white light source is used and split into a number, typically three, basic colors or color ranges. Very often these correspond with the three primary colors red, green and blue. As the need for high quality color projection image is rapidly increasing and as the human eye is very sensitive for color differences, the optics used to split the light of a white light source will play an important role and should be designed carefully. Not only are the sizes of the projection screens continuously increasing, so that small errors in color projection become relatively more important, the human eye is also very sensitive to color differences. Therefore, it is an essential feature to obtain an optimum color projection. This includes both a high contrast and a good color homogeneity, i.e. a reduced color shift.
The problem of improving contrast is discussed in U.S. Pat. No. 5,934,778, which describes an optical system for high contrast projection. As many systems suffer from color shifts created by incident angle dependency for dichroic prisms and from unwanted reflection of certain light colors, U.S. Pat. No. 5,934,778 suggests inclusion of additional filters to reduce the influence of unwanted reflected color beams on the contrast of other colors. It is alleged that this allows higher contrast images than with conventional systems.
One way to improve the color uniformity and homogeneity is providing equal path lengths for the different colors. This allows uniform magnifications of the illumination pattern for each of the three color channels. If illumination patterns with generally the same intensity uniformity profiles are formed and these are superposed, this allows improved uniformity of the color image.
U.S. Pat. No. 6,532,044 describes an electronic projector combining multiple projection lens assemblies with equal color component optical path lengths to provide improved display images and a compact arrangement. The document discusses the use of separate projection lens assemblies for each of the basic colours which are arranged in a non-linear, close-packed arrangement to receive the different color components of light. The arrangement does not comprise an X-cube but has different dichroic mirrors. Therefore, equal path lengths can be obtained using mirrors and dichroic mirrors, as suggested in U.S. Pat. No. 6,532,044. Nevertheless, as no convergence of the different color beams is performed, the system is relatively complex compared to conventional three imager parallel color systems. Three projection lens assemblies are provided, which makes the quality for color imaging strongly dependent of the quality and outlining of the different projection assemblies.
DE4313139 describes a projection system having a metal halide lamp emitting light in its three primary colors by a system of filters and mirrors. The system is built such that the three optical paths from the source to the LCD panels are all equal in length. The advantage of the system is that there is an improved projection by providing a shorter distance between the focus and the rear of the projection lens and that fewer dichroic mirrors are needed in the color synthesizer. Nevertheless, the system does not include an X-cube. This leads to additional problems with color shift and light intensity of the different colors in the illumination beam as after modulation the different color beams need to pass different and a different number of dichroic mirrors in order to form one single illumination beam.
In similar way, EP 0 458 687 describes a system having a large degree of compactness and a good resolution power. The system is based on a mirror system whereby each of the primary colors is modulated by a light modulating means. This system also does not comprise an X-cube and therefore recombination of the different color beams needs to be performed by different dichroic mirrors, leading to additional problems with color shift and light intensity of the different colors.
None of the above documents describes optimization of the color uniformity by providing equal path lengths for the different colors for systems using an X-cube for the splitting and/or recombination of the light.